IP Multimedia Subsystem (IMS) is the technology defined by the Third Generation Partnership Project (3GPP) to provide IP Multimedia services over mobile communications networks. IMS is specified in, amongst others, 3GPP TS 22.228, version 8.5.0. IMS provides key features to enrich the end-user person-to-person communication experience through the integration and interaction of services.
The IMS makes use of the Session Initiation Protocol (SIP) to set up and control calls or sessions between communication devices (also known as user terminals (UEs)) or between communication devices and application servers (AS). The Session Description Protocol (SDP), carried by SIP signalling, is used to describe and negotiate the media components of the session. Whilst SIP was created as a user-to-user protocol, IMS allows operators and service providers to control subscriber access to services and to charge subscribers accordingly. Telephones using this protocol are referred to as SIP telephone.
A SIP telephone may be any telephone tool, including a software tool, also referred to as a client, running on a computer, using an IP telephony signalling protocol. The SIP telephone may be used for any media type, including voice data, video data etc. The SIP telephone is typically location bound, but may also use a wireless internet connection.
Nowadays, subscribers may have more than one telephone device in use, such as a wireline/wireless SIP telephone and a (cellular) mobile telephone. The term mobile telephone is used in this text to denote any telephone that is not location bound, i.e. a mobile telephone using a cellular telephone network, enabling the mobile network to be mobile. A mobile telephone may for instance use the GSM-network or the UMTS-network. The term mobile telephone does not comprise cordless telephones, which are bound to a location, i.e. are bound to a Wireless Local Area Network (WLAN) base station.
The SIP telephone will typically be used at a specific location, such as at home or at the office, whereas the mobile telephone will typically be used when being somewhere else than at the specific location of the SIP telephone. When the subscriber establishes or receives a call, he/she will use the device that is most suitable at that moment or the most cost-efficient. When a subscriber receives a call, the incoming call may be alerting on the SIP telephone as well as on the mobile telephone, providing the subscriber with the option to choose which one to use. Reasons for using the SIP telephone may be:                SIP telephone may support a (more user-friendly) handsfree function than the mobile telephone;        SIP telephone has a lower tariff than a mobile telephone.        
The preferred device (SIP telephone or mobile telephone) may change during the call. For instance,                a subscriber may want to change from SIP telephone to mobile telephone when the subscriber wants to walk away from the SIP telephone,        a subscriber may want to change from mobile telephone to SIP telephone when the subscriber arrives at the location of the SIP telephone.        
According to the prior art transferring a communication session from a first connection between a first client in a first communication device (e.g. SIP-telephone) and a remote client to a second connection between a second client in a second communication device (e.g. mobile telephone) and the remote client, the communication session comprising a media path and a signalling path may be done using the service Explicit Call Transfer (ECT), as specified for and performed by the Global System for Mobile Communication (GSM) and Universal Mobile Telecommunication System (UMTS) network. ECT, however, has the disadvantage that the remote party is temporarily put on hold. In addition, when a subscriber wants to use ECT to transfer a call to one of his/her other communication devices, the subscriber in fact imitates a call to his/herself, which may result in unpredictable service behaviour. The unpredictability of such situation may manifest itself e.g. through the execution of terminating call services associated with the subscriber. Alternatively, the system would not allow ‘calling yourself’. In general, ECT is not suitable for device transfer, as ECT essentially entails establishing another call.
Also, initiating a call to one of his/her other communication devices involves inputting a telephone number or the like (e.g. using a memory button), which may be considered cumbersome.
According to the prior art, the IMS network may comprise application servers which are arranged to host and execute services. Application servers may be located in the home network (possibly having access to a Home Subscriber Server (HSS), further explained below) or may be located outside the home network.
The register HSS is in fact a database comprising information about the IMS network subscribers. The HSS may comprise information about the subscriber's subscription, i.e. user profiles. Also, the HSS plays an important role when a user terminal registers to the IMS network, as it authenticates and authorizes the user terminal. The HSS will be explained in more detail below.
An example of such a service is the Voice Call Continuity (VCC); see 3GPP TS 23.206 v7.4.0, which comprises the anchoring in IMS of a call established to/from the subscriber. Anchoring in IMS for the call is, according to VCC, done at a VCC application server (AS). The VCC-AS functions as a link between the access leg for the call and the IMS network.
VCC is designed to facilitate access transfer using a single, multi access device, e.g. to change from SIP over WLAN access to GSM access. It should be emphasised that VCC can not be used for transferring a call between different devices.
When the VCC device detects that access transfer is required, e.g. because the WLAN radio signal is fading, the VCC device initiates transfer of the call from WLAN to GSM. While WLAN was used, the call was established with SIP. When GSM is used for access, protocols such as Direct Transfer Application Part (DTAP) and Integrated Services Digital Network User Part (ISUP) are used for call establishment. After the access transfer, the access leg between the device and the network has changed from SIP (over WLAN) to GSM. Within the IMS network, the call is not affected. Although the VCC device has two radio interfaces (typically WLAN and Time Division Multiple Access (TDMA)) and corresponding protocol stacks (SIP for WLAN and DTAP for TDMA), it has a single user interface. The access transfer can therefore be hidden for the subscriber.
The prior art solutions do not allow to seamlessly transfer communication between two different communication devices, such as a SIP telephone and a mobile telephone.